This invention relates to a span adjusting method and apparatus for applying a known load to a weighing machine, subjecting the value of an output signal from the weighing machine to an analog-to-digital, or AD, conversion, and effecting an adjustment in such a manner that the digital value resulting from the AD conversion is rendered into a digital value conforming to the applied load.
The span adjustment mentioned above will now be described in greater detail. Assume that a digital value commensurate with a load of 100 g is determined beforehand to be "1000", and that the AD-converted output signal from an as yet unadjusted weighing machine, to which the load of 100 g has actually been applied, is a value of "1010". In such a case it is necessary to effect an adjustment so that the digital output value is brought from "1010" to the desired value of "1000". This adjustment is referred to as a span adjustment.
A weighing machine employed in a combinatorial weighing system or the like includes a weight sensor and a weighing hopper, the former comprising a load cell. Articles introduced into the weighing hopper are weighed by the weight sensor, which has an output which is indicative of the measured weight, and which is applied to a computerized combinatorial control unit through an amplifier an AD converter. The combinatorial control unit is operable to form combinations of weight values obtained from a plurality of the aforementioned weighing machines constituting the combinatorial weighing system, to calculate the total weight of each combination, to obtain a combination (referred to as an "optimum combination"), whose total weight value is equal to a target weight value or closest to the target weight value within preset allowable limits, to discharge the articles from the weighing hoppers of the weighing machines corresponding to the optimum combination (whereby these weighing hoppers are left empty), to resupply the emptied weighing hoppers of these weighing machines with articles in order to prepare for the next weighing cycle, and to repeat the foregoing steps in similar fashion to carry out a continuous automatic weighing operation.
The load cell and amplifier mentioned above have characteristics that vary with temperature. Therefore, unless certain measures are taken, the weight signal output from the weighing machine fluctuates due to a change in zero point and span settings, thereby making it impossible to maintain good weighing precision. Accordingly, in the prior art, a variable resistor for zero-point and span adjustment is provided for each and every weighing machine, and each resistor is controlled manually to compensate for the change in zero point and span ascribable to variation in the characteristics of the load cell and amplifier. However, since this conventional zero-point and span adjustment is performed by manual control of the variable resistors, adjustment requires considerable time and effort, particularly when there are a large number of weighing machines. The present applicant has proposed a method of automatically adjusting zero point in copending application Ser. No. 605,376 filed Apr. 27, 1984. The present invention is directed to a system for span adjustment.